Business Segments (Textual) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Management fees expense	$ 11.5	$ 9.3	$ 8.5
Customer concentration risk | Sales revenue, net | Choctaw Generation Limited Partnership | NACoal
Segment Reporting Information [Line Items]
Revenue from major customer, percentage	69.00%	57.00%	39.00%
Customer concentration risk | Sales revenue, net | Cemex | NACoal
Segment Reporting Information [Line Items]
Revenue from major customer, percentage	16.00%
Customer concentration risk | Sales revenue, net | Alabama Coal Cooperative | Reed Minerals
Segment Reporting Information [Line Items]
Revenue from major customer, percentage		16.00%	27.00%
Customer concentration risk | Sales revenue, net | Wal-Mart | HBB
Segment Reporting Information [Line Items]
Revenue from major customer, percentage	32.00%	32.00%	33.00%
Customer concentration risk | Sales revenue, net | Amazon | HBB
Segment Reporting Information [Line Items]
Revenue from major customer, percentage	10.00%
Customer concentration risk | Sales revenue, net | Five Largest Customers | HBB
Segment Reporting Information [Line Items]
Revenue from major customer, percentage	54.00%	52.00%	56.00%